1. Field of the Invention
The present invention relates to a lithographic system, a method for manufacturing a device, and an apparatus and method for optimizing setpoint data for an array of individually controllable elements.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
A chip layout tool or similar design utility can be used to define a target device structure that a user wishes to generate on a substrate. The target device structure can be output from such a utility in a standard vector-based format such as GDSII or OASIS.
The target device structure defined in the vector-based format output file has infinitely sharp edges and corners. It does not take into account limitations in the projection system that will project the patterned radiation beam, nor in the post-processing steps that will be used to process the irradiated substrate in order to form a device structure.
Diffraction effects will cause the projection system to have a restricted (spatial) frequency response, causing in particular a low pass-filter behavior (in the spatial frequency domain). The post-processing steps can have a similar effect, acting broadly to blur the device structure relative to the dose-to-clear contour defined by the radiation dose pattern exposed onto the substrate.
One approach for correcting for these effects is by modifying the vector-based format output file to incorporate corrective features, also known as “optical proximity corrections” (OPCs).
The OPCs can be derived by theoretical modeling of the system. This can include calculation of the device structure that would result from use of a mask corresponding to the vector-based format output file with a particular projection system and post-processing regime. OPCs can then be added to the mask and the calculation repeated iteratively for different OPCs until a difference between the calculated device structure and the vector-based format output file falls within acceptable bounds.
Where a maskless system is to be used to emulate a physical mask, the same OPCs as were calculated for the mask-based system can be used, but care needs to be taken to ensure the array(s) of individually controllable elements in the maskless system behave(s) in the same way as the physical mask. Due to physical differences in the array(s) of individually controllable elements and the way the array(s) of individually controllable elements interact with the radiation beam in comparison with physical masks, accurate emulation can be difficult to achieve.
Therefore, what is needed is a system and method to improve the accuracy of device structures formed by maskless lithography systems.